A MIMO wireless communication system (see FIG. 1) is one which comprises a plurality of antennas 10 at the transmitter 11 and one or more antennas 12 at the receiver 13. The antennas 10, 12 are employed in a multi-path rich environment such that due to the presence of various scattering objects (buildings, cars, hills, etc.) in the environment, each signal experiences multipath propagation. Thus a cloud shape 14 is shown in FIG. 1 to represent the scattered signals between the transmit and receive antennas. User data is transmitted from the transmit antennas using a space-time coding (STC) transmission method as is known in the art. The receive antennas 12 capture the transmitted signals and a signal processing technique is then applied as known in the art, to separate the transmitted signals and recover the user data.
MIMO wireless communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved compared with previous systems in the respect that higher data rates can be obtained. The multipath rich environment enables multiple orthogonal channels to be generated between the transmitter and receiver. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth. Consequently, higher spectral efficiencies are achieved than with non-MIMO systems.
One problem with existing MIMO systems concerns the large size of the transmit and receive antenna arrays. Previously, MIMO transmit and receive antenna arrays have used spatially diverse antenna arrays. That is, the spacing between the individual antenna elements is arranged to be large enough such that decorrelated spatial fading is obtained. This is necessary in order to prevent the number of orthogonal channels from being reduced. That is, if the fading characteristics between antenna elements is similar (correlated) then the number of orthogonal channels that can be realised is reduced. For example, for rooftop installations, or antennas on towers, separations of up to 20 wavelengths may be required to achieve decorrelated fading due to the low angle spread of the multipath.
Another problem with existing MIMO systems is that they are designed for use in environments where scattering occurs rather than for line of sight situations. However, line of sight situations arise in many circumstances, such as communication between portable wireless devices that are close together and communication in fixed wireless access systems in which directional arrays are used at subscriber premises. This means that it has not previously peen possible to realise the potential capacity available from MIMO systems in such line of sight situations.
The spatial diversity arrangement in previous MIMO systems has also meant that such systems are incompatible with multi-beam antenna arrangements which require closely spaced antenna arrays with no spatial diversity. A multi-beam antenna arrangement is one in which a plurality of closely spaced antenna elements 21 (see FIG. 2) is used together with a beamformer 20 to form two or more directional antenna beams 23. Data to be transmitted enters on inputs 24 and is transmitted to a plurality of user equipment terminals 22. The antenna element spacing is such that no spatial diversity is present and is typically achieved with an antenna spacing of half a wavelength. By using multiple directional antenna beams in this way interference between the beams is reduced and thus downlink capacity increased. That is, the number of user equipment terminals that can be supported by a single basestation comprising the antenna array 21 is increased. This differs from a MIMO system such as that illustrated in FIG. 1, where downlink capacity is increased for a particular user or plurality of users by increasing the data rate to those users.
An object of the present invention is to provide a MIMO wireless communications system which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.